The term “stent for angioplasty” is intended to indicate generally devices intended for endoluminal application (for example, within a blood vessel) in association with the technique of percutaneous transluminal coronary angioplasty, or PTCA, usually effected by catheterisation of a stenotic site.
Expanding the stent at the site causes the lumen to expand giving rise to the consequent elimination of the stenosis, and the local support of the lumen by the stent, which is left in place expanded, avoids restenosis of the treated site due to the subsequent relaxation of the blood vessel wall. The use of a substantially similar structure for deploying vascular grafts and fixing them in place has already been proposed in the art: naturally, this possible extension of the field of application should be seen as included within the ambit of the present invention.
For a general review of vascular stents, reference may usefully be made to the work “Textbook of Interventional Cardiology” edited by Eric J. Topol, W.B. Saunders Company, 1994 and, in particular, to section IV of volume II, entitled “Coronary Stenting”.
Many patent documents have addressed this problem, for example, U.S. Pat. No. 4,776,337, U.S. Pat. No. 4,800,882, U.S. Pat. No. 4,907,336, U.S. Pat. No. 4,886,062, U.S. Pat. No. 4,830,003, U.S. Pat. No. 4,856,516, U.S. Pat. No. 4,768,507 and U.S. Pat. No. 4,503,569.
The implantation of these devices, which is a factor in the treatment of various cardiac diseases, may require, or at least gain particular benefit from the possibility of being able to administer at the stent-implantation site agents or active principles (the two terms being used below in an equivalent sense) having various end purposes: they may, for example, be antithrombogenic agents or, more generally, agents for directly resisting restenosis of the treated site due to the formation of deposits, tissue proliferation, etc. In relation to this, reference may usefully be made to the following works:
“Local Drug Delivery: The Development of a Drug Delivery Stent” by Richard Stack, The Journal of Invasive Cardiology, Vol. 8, n. 8, October 1996, pp 396-397;
“Local Intraluminal Infusion of Biodegradable Polymeric Nanoparticles” by Louis A. Guzman et al., Circulation, 1996; 94; pp 1441-1448;
“Local Angiopeptin Delivery Using Coated Stents Reduces Neointimal Proliferation in Overstretched Porcine Coronary Arteries” by Ivan De Schreerder et al., the Journal of Invasive Cardiology, Vol. 8, n. 8, October 1996, pp 215-222.
Many applicational problems arise from this mode of operation, mostly related to the specific solutions adopted. For example, the problem exists of avoiding the agent or agents intended for administration in the zone of the stent being delivered or transported to different areas where they may have negative or damaging effects. Other problems may arise, for example, in ensuring the permanence and the gradual release over time of active substances capable of being, as it were, washed away by the blood passing through the stent.
These problems cannot themselves be solved or avoided by recourse to other solutions such as radioactive stents or so-called biodegradable stents, as illustrated, for example, in the work “Biodegradable Stents: The Future of Interventional Cardiology?” by M. Labinaz et al; Journal of International Cardiology, Vol. 8, n. 4, 1995, pp 395-405. Radioactive stents publicly proposed so far give rise to other problems related essentially to the fact that, in most cases, their use assumes the typical features of radiotherapy and/or nuclear medicine. The main disadvantage of biodegradable stents is that, at least in the long term when the stent has completely or substantially degraded, there is a reduction in the mechanical support of the blood vessel wall against the risk of collapse.
As a further solution for administering various kinds of active principle at the stent-implantation site a solution has recently been proposed in which at least a portion of the surface of the body of the stent (or implantation device in general) is coated with a receptor capable of binding with a ligand formed by combining an active principle with a substance capable of binding to the receptor.
In order for this new solution to be fully beneficial, that is, so that it can also be used with more conventional techniques for effective topical administration of the active principles, it appears important to obtain a good adhesion and/or retention on the stent of the substance or substances with which these active principles are associated and/or are intended to be associated.
In relation to this it is therefore necessary to take account of various concomitant factors which often oppose one another.
In a significant number of applications it is important that the active principles are present mainly, although not exclusively, on the outer surface of the stent. Conversely, it is usually desirable that the inner surface of the stent itself is as inert as possible, that is, both from the chemical point of view and from the point of view of the possible mechanical anchorage of possible deposits.
This is the reason why currently available vascular stents are subjected to a polishing process, intended to make the surface of the stent (both inside and outside) very smooth. In relation to this, it is also possible to coat the stent with a layer of biocompatible material, such as a biocompatible carbon material (deposited, for example, using sputtering techniques), so as to confer a high degree of hemocompatability on the whole stent. Adopting this technique for the deposition of such a layer, given the very small dimensions of a stent for angioplasty, means that it is practically impossible to limit the deposition to just the inside surface of the stent. Consequently therefore, the entire surface of the stent is coated with a layer which, by its nature, makes the deposition of substances on the stent itself, in fact, impossible.
A further factor should not be forgotten: a stent for angioplasty is by its nature a heavily apertured structure, usually a mesh-like structure in which, especially in the radially-extended position, the effective surface intended to come into contact with the blood vessel wall is a small fraction of the theoretical tubular surface area defined by the outside of the stent itself. In other words: even by putting the other problems described above to one side, there is very little available surface on the stent for carrying the active principles intended for local delivery.
The object of the present invention is that of resolving the disadvantages described above.
In particular, the solution according to the invention, having the characteristics referred to in the following claims, enables the selective application, specifically to the outer surface only of the stent, of a completely effective quantity of active principle (either directly or in the form of a receptor capable of binding with a ligand carrying the active principle) without by this losing the possibility of having a very smooth surface, at least inside the stent, even if clad with coatings such as haemocompatible carbon coatings.